A source close to the McConnell campaign told NBC News that FBI agents were at campaign headquarters for about an hour on Wednesday, during which point the campaign handed over pertinent information to agents. The source would not specify the information provided to investigators.

"They tell us that they’re running down some leads," Benton said. "For various reasons they need to be very cautious about what they share with me and then what I’m allowed to share on the public side, I can’t comment any further, but this is an ongoing criminal investigation."

The progressive magazine Mother Jones published a two-minute audio clip featuring McConnell and top campaign staff discussing opposition research into potential Democratic challengers, including the actress Ashley Judd. The McConnell campaign staff is heard on tape discussing Judd's faith and mental health history as potential sources of criticism during a campaign.

(Mother Jones has denied obtaining the audio through any illicit means.)

The McConnell campaign has asserted it was the victim of a bugging effort, though they have not produced any hard evidence yet to suggest that. However, today's visit by the FBI to campaign headquarters suggests the agency is taking the matter seriously at the moment.

"This is Gestapo-kind of scare tactics, and we're not going to stand for it," Benton said.